1. Field of the Invention
The invention relates in general to power line carrier communication systems, and more specifically to power line carrier communication systems which utilize the distribution network for communication.
2. Description of the Prior Art
Power line carrier communication has been used for many years over the high voltage transmission lines which extend from the power generating site to the distribution substations, for supervisory control purposes. The transmission line is a homogeneous medium for communication signals as it extends for miles without interruptions of the type which would disturb the communication signal.
In recent years the electric utilities have expressed an interest in power line carrier for automating the distribution of electricity, i.e, such tasks as performing load continuity checks, switching, selective load control, automatic reading of utility meters, and the like. The primary and secondary distribution network, however, is anything but homogeneous from the viewpoint of a carrier current signal, which is typically in the frequency range of 30 kHz to 300 kHz.
A major disadvantage in implementing a large scale distribution power line carrier communication system is the difficulty to predict or calculate signal levels at all points of the distribution network. This is largely due to the variations of signal propagation caused by variations in the geometry of conductor spacing, and unequal loading of the three-phase distribution lines by transformers, branches and taps.
The problems associated with distributing carrier currents within a large building were dealt with in U.S. Pat. No. 2,743,434 by applying the carrier current signal equally to all of the single phase conductors and the neutral conductor within a common conduit, to cause them to function as a single conductor within the conduit. While this arrangement may be suitable for communications within a building, applying the carrier current signal to all of the conductors of a polyphase primary distribution system provides little or no improvement in predicting signal levels at the various points of the distribution system, as the propagation mode is converted to a plurality of modes at the first line discontinuity.